1. Field of the Invention
The present invention relates to a sliding material the base resin of which is polytetrafluoroethylene (hereinafter referred to as PTFE).
2. Description of the Related Art
PTFE is a suitable resin for a sliding material because of its low coefficient of friction and its excellent self-lubricating property, while the wear resistance thereof is not sufficient. Accordingly, there is widely used a sliding material the wear resistance of which is enhanced by mixing a PTFE resin with metal particles, another resin, or the like.
For example, JP-B-39-16950 describes a bearing material wherein a porous copper alloy sintered on a back metal is impregnated and coated with a resin made from PTFE which includes, by volume, 16 to 24% lead particles. Because of including the lead particles, the PTFE resin is improved in wear resistance, particularly under the dry condition.
However, in recent years, a sliding material which does not include the lead particles also has been desired in view of environmental problems.
Accordingly, in order to develop a sliding material substituting for the conventional sliding material including the lead particles, the present inventors have repeated many trials and errors and as a result of those, found that a sliding material comprising PTFE which includes bismuth particles can substitute for the conventional sliding material including lead particles in view of properties regarding the coefficient of friction and wear resistance, so that the invention had been completed.
According to the invention, in order to achieve the above object, there is provided a sliding material comprising PTFE which includes, by volume, 3 to 40% bismuth particles but no lead particles. Because of not including the lead particles, it is possible to provide a sliding material harmless to the environment.
In case that the content of the bismuth particles is less than 3 volume %, it is not possible to obtain the effect sufficiently improved in wear resistance. On the other hand, in case that the content is 10 to 40 volume %, it is possible to obtain excellent wear resistance, however, in case that the content exceeds 30 volume %, the wear resistance is gradually deteriorated. Thus, the content of the bismuth particles may be from 3 to 40 volume %, preferably from 10 to 30 volume %, and more preferably from 15 to 20 volume %.
The bismuth particles included in the resin are preferably dispersed therein, however, if the particles are too fine, it is difficult to disperse them in PTFE. Thus, it is desirable that the size of the bismuth particles is about 1 to 50 xcexcm.
Since bismuth alloy particles including a metal such as silver, tin, zinc or indium are harder than pure bismuth particles, the bismuth alloy particles can further improve the wear resistance of the sliding material. The content of the metal is preferably from 0.5 to 30 weight %, and more preferably from 5 to 15 weight %.
The sliding material may be used alone for configuring a plain bearing, however, it may be used also for coating a back metal of a plain bearing so as to enhance the mechanical strength thereof. Moreover, after sintering metal powder on the back metal of the plain bearing, the metal powder layer may be impregnated and coated with the sliding material, so that the adhesiveness of the resin layer is improved. Furthermore, a wire netting member or an expanded metal member may be filled and coated with the sliding material to constitute a plain bearing.
By using the resin comprising PTFE which includes, by volume, 3 to 40% bismuth particles, there is provided a sliding material which is excellent in bearing properties such as a coefficient of friction and wear resistance, and harmless to the environment because of not including the lead particles.
Moreover, the bearing properties such as a coefficient of friction and wear resistance can be further improved by replacing the bismuth particles with the bismuth alloy particles.
By using the sliding material according to the invention for a plain bearing, it is possible to provide a plain bearing improved in thermal conductivity, load carrying capacity and wear resistance.
With reference to the attached drawings, preferred embodiments of the invention will be described herein below.